Polypropylene resins have excellent properties and are used in various fields including automobile parts, machinery parts and electric parts.
Recent automobile parts have been strongly required to be reduced in weight for fuel efficiency, and therefore foamed articles have come into use as automobile interior materials. For further weight reduction in automobiles, it is demanded that articles having an equivalent performance to the conventional level should be manufactured from a less amount of resin. In other words, injection-molded foams with a higher expansion ratio are strongly desired.
Reducing the amount of resin in the foaming results in a thin thickness of the resin placed in a space of a mold (hereinafter, the cavity). As a result, the foam production is very difficult and satisfactory foams cannot be obtained.
In a known injection foaming process, pellets of a raw material thermoplastic resin are directly sprinkled or masterbatched with thermally decomposable chemical foaming agents such as azodicarbonamides, sodium bicarbonate and citric acid. Since the chemical foaming agents are easily available and are usable with a conventional inline injection molding apparatus, they are widely used in injection foaming. However, when the amount of chemical foaming agents mixed with the pellets is increased to expand the resin at a high ratio, the injection foaming tends to result in bad appearance of the foams. In detail, when a chemical foaming agent as a foaming component is added in excess of 5 parts by weight relative to 100 parts by weight of a thermoplastic resin, a higher proportion of the chemical foaming agent is not involved in the foaming and remains unexpanded, and such unexpanded chemical foaming agent present on the surface of the foams deteriorates the appearance. Further, increasing the amount of chemical foaming agents achieves only a limited level of expansion ratio.
Instead of use of the chemical foaming agents, Patent Document 1 discloses that a gaseous or supercritical physical foaming agent is injected at a position on a cylinder of an injection molding machine. This method has enabled the production of foams with good appearance, but costs are increased because the injection molding machine has a specific structure and is complicated in particular when using supercritical physical foaming agent. Although the expansion ratio can be increased by injecting a large amount of the gas, a great number of swirl marks or fine dimples are often caused on the surface. Accordingly, the injection molding with the physical foaming agents is not suited in the industrial fields where the expansion ratio is high to achieve improved rigidity, and the use thereof is rather limited to injection foaming in which the improved size accuracy or cycle reduction is targeted without increasing the expansion ratio.
Accordingly, there has been a need for polypropylene resin compositions that are suited for the production of injection-molded foams and can give foams with a reduced thickness, excellent appearance and superior mechanical properties such as impact resistance.
Patent Document 1: JP-A-2002-79545